The present invention relates to a method for controlling the output power of optical amplifiers, particularly in nodes in a network, and to a node in a network.
Wave Division Multiplexing (WDM) is a technology allowing the transmission of a number of optical signals through an optical fibre using different separated light wavelengths. In this way the information carrying capacity may be increased significantly. The capacity depends on the bandwidth of the channels and the number of used wavelength channels. The signal at each wavelength travels through the fibre irrespectful of the other signals, so that each signal represents a discrete channel with large bandwidth.
When signals are sent long distances the signals have to be regenerated or optically amplified at intervals. An optical amplifier may comprise an erbiumdoped fibre that is being pumped by a high power semiconductor laser, here called xe2x80x9cpump laserxe2x80x9d for short.
The optical amplifier differs from the electrical amplifier in that the optical amplifier strives to keep a constant output power, since they normally are in a more or less saturated state, while the electrical amplifier has constant gain.
That the output power is constant gives problems if the number of channels are changedxe2x80x94by purpose or by a fault. It may therefore be desirable to be able to control the output power.
It is earlier known to control the gain in an optical amplifier by using pilot tones, see EP 0 637 148 A1. An identifying pilot tone is being modulated on each multiplexed wavelength. Each amplifier in the system decides from the pilot tone the total number of sent wavelengths and therefore gives a corresponding control of the gain of the amplifier. Either feedforward or feedback is used. In the document is also hinted that in the addition to the count of the number of channels the amplitude of the pilot tones could be measured to obtain a better control.
It is also earlier known, see GB 2 294 170, to control the amplifier in different ways by measuring total optical effect on the output of the amplifier, transforming it to a voltage and comparing it with a reference voltage. This gives a feedback signal that controls the amplifier. As an addition the number of channels may be counted and the control may be adjusted accordingly.
Further it is known, see U.S. Pat. No. 4,991,229, to control an optical amplifier by measuring the power in only one wavelength channel. This is done by filtering out said channel with the aid of a WDM-coupler and then detecting the channel. Otherwise, feedback is made on a similar way as above.
In U.S. Pat. No. 5,113,459 is shown an idea of using a special channel for optical service signalling.
The problems with modulating identifying pilot tones on each multiplexed wavelength in order to be able to count the wavelengths is that it is complicated, slow and expensive.
Another problem is that it also influences the transmission performances in a negative way.
The purpose of the present invention is to provide a simple and effective power per channel control of amplifiers in nodes in a network forming a bus. In order to do that a simple, fast and cheap way of counting the channels entering each amplifier is accomplished.
The counting of channels is achieved by using a separate channel for signalling between the nodes, wherein the number n of channels is transmitted between the nodes. An alternative would be to use a separate electrical network, which however is slower.
The first node of the bus sets the number n to the number of active transmitters in the node. The rest of the nodes updates the number n by adding the number of channels that they are adding to the bus and by subtracting the number of channels that they are blocking or terminating.
In an embodiment of the invention large and fast input power changes are detected, which may be due to a fault, which in its turn would make the number n coming in to the node outdated. In that case the updating of the number n is inhibited until a new and trustable number n is received.
The advantages with the invention is that it is simple, fast and cheap.